This invention relates generally to gas turbine engines and more particularly to a free vortex air oil separator for recovering oil used to lubricate and cool bearings used in gearboxes and other components of a gas turbine engine.
Gas turbine engines typically include a core having a compressor for compressing air entering the core, a combustor where fuel is mixed with the compressed air and then burned to create a high energy gas stream, and a high pressure turbine which extracts energy from the gas stream to drive the compressor. In aircraft turbofan engines, a low pressure turbine located downstream from the core extracts more energy from the gas stream for driving a fan. The fan usually provides the main propulsive thrust generated by the engine.
Bearings are used in the engine to accurately locate and rotatably mount rotors with respect to stators in the compressor and high and low pressure turbines of the engine. The bearings are enclosed in oil-wetted portions of the engine called sumps.
In order to prevent overheating of the bearings, lubricating oil and seals must be provided to prevent the hot air in the engine flowpath from reaching the bearing sumps, and lubricating oil flows must be sufficient to carry away heat generated internally by the bearings because of their high relative speed of rotation and heat load from environment.
Oil consumption arises from the method used to seal the bearing sumps. The sealing method makes it necessary for an air flow circuit to exist that flows into and out of the sumps. This flow ultimately contains oil that is unrecoverable unless adequately separated and delivered back to the sumps. In one particular configuration the forward engine sump is vented through the forward fan shaft and out of the engine through a center vent tube. Once the air/oil mixture exits the sump, it swirls, depositing oil on the inside of the fan shaft. Oil that is contained in the air/oil mixture is lost when it is unable to centrifuge back into the sump through the vent hole due to rapidly escaping vent air.
Some conventional designs allow for oil recovery by using weep holes, which are passages whose function is to provide a dedicated path for oil to re-enter the sump, integrated into the forward fan shaft design. In other conventional designs, the fan shaft has no dedicated weep holes, only vent holes. Some conventional designs use a weep plug in a rotating shaft that injects the air-oil mixture radially into a chamber for separating the oil and air, and routes the separated oil through a passage in the weep plug. The weep plug allows the air-oil mixture to radially enter a separator cavity through a central passage in the weep plug. As the air-oil mixture swirls down to a lower radius centrifugal forces drive the more massive oil particles back to the inside diameter of the shaft, while the air escapes through the vent exit. Other conventional designs use holes in the engine high pressure turbine (HPT) or low pressure turbine (LPT) shafts or gearbox shafts for oil separation. In some conventional designs, these holes in the shaft may be filled with honeycomb or sponge-like substances to enhance the oil separation. Conventional air-oil separators commonly used in gearboxes and gas turbine engines are either the forced-vortex static type or rotating mechanical type. The forced-vortex static separators are typically part of the over-board vent circuit or oil reservoir assembly mounted in a static structure and these systems centrifuge out the oil from air-oil mixture in the forced vortex. Conventional rotating mechanical separators typically use the main engine or gearbox shaft holes for mechanical separation. The gearbox separators are either mounted externally on a pad of the gearbox or use the internal gear shaft holes to mechanically separate air and oil. In some cases these holes are filled with honeycomb or an open cell porous substance to enhance this mechanical separation. These devices are typically limited in their separation strength and thus efficiency. Air-oil separation is particularly poor in these conventional designs in cases where the axial distances are short in the sump region. It has been found that without adequate dwell time for vortex motion, oil separation from the air-oil mixture will be poor. The bearings normally operate at high loads and speeds and, as a result, usually run at high temperatures. The lubrication oil that is supplied provides cooling to the bearings. However the air-oil mixture that is formed in the sump acquires a high temperature. It is more difficult to separate the oil from the air-oil mixture at higher temperatures.
It is desirable to have an air-oil separator that is effective in removing oil in engine systems and gearboxes which have sumps that are axially short. It is desirable to have a free vortex air-oil separator system with increased dwell time at larger radii for the vortex motion and higher vortex rotational speed for the air-oil mixture. It is desirable to have a free vortex air-oil separator that can cool the air-oil mixture prior to separating the oil from the air-oil mixture.